Microbial and genetic ecology of tropical Vertisols under intensive chemical farming

There are continued concerns on unscientific usage of chemical fertilizers and pesticides, particularly in many developing countries leading to adverse consequences for soil biological quality and agricultural sustainability. In farmers' fields in tropical Vertisols of peninsular India, "high" fertilizer and pesticide usage at about 2.3 times the recommended rates in black gram (Vigna mungo) did not have a deleterious effect on the abundance of culturable microorganisms, associative nitrogen fixers, nitrifiers, and 16S rRNA gene diversity compared to normal rates. However, "very high" application at about five times the fertilizers and 1.5 times pesticides in chilies (Capsicum annuum) adversely affected the populations of fungi, actinomycetes, and ammonifiers, along with a drastic change in the eubacterial community profile and diversity over normal rates. Actinobacteria were dominant in black gram normal (BG1) (47%), black gram high (BG2) (36%), and chili normal (CH1) (30%) and were least in chili very high (CH2) (14%). Geodermatophilus formed 20% of Actinobacteria in BG1 but disappeared in BG2, CH1, and CH2. Asticcacaulis dominated at "very high" input site (CH2). Diversity of nitrogen fixers was completely altered; Dechloromonas and Anaeromyxobacter were absent in BG1 but proliferated well in BG2. There was reduction in rhizobial nifH sequences in BG2 by 46%. Phylogenetic differences characterized by UniFrac and principal coordinate analysis showed that BG2 and CH2 clustered together depicting a common pattern of genetic shift, while BG1 and CH1 fell at different axis. Overall, there were adverse consequences of "very high" fertilizer and pesticide usage on soil microbial diversity and function in tropical Vertisols.